1. Field of the Invention
The present invention relates to a thermoelectric conversion material comprising a metal oxynitride, and more particularly, to a metal oxynitride having superior characteristics as an n-type thermoelectric conversion material, and to an n-type thermoelectric conversion material, a thermoelectric conversion element, and a thermoelectric power generating module comprising such metal oxynitride.
2. Description of the Related Art
In this country, the effective energy yield from primary supply energy is merely of about 30%, with about 70% of that energy being dissipated into the atmosphere as heat. Also, most of the heat generated as a result of combustion in factories, waste incineration plants and the like is dissipated into the atmosphere without being converted into some other energy. Mankind wastes thus thermal energy in a huge scale while extracting only a small amount of energy from activities such as burning of non-renewable petroleum fuels.
In order to increase energy yields, this thermal energy wasted into the atmosphere must be exploited somehow. Thermoelectric conversion, in which thermal energy is converted directly into electrical energy, is an effective means for attaining that goal. Thermoelectric conversion is an energy conversion method based on the Seebeck effect, wherein electricity is generated through thermoelectric power that arises as a result of a temperature difference between two ends of a thermoelectric conversion material.
In thermoelectric generation, power can be obtained, for instance, by simply arranging the end of a thermoelectric conversion material at a high-temperature portion warmed through waste heat, arranging the other end of the thermoelectric conversion material in the atmosphere (room temperature portion), and connecting both ends with a conductive wire. That is, mobile components such as motors, turbines or the like are unnecessary in an ordinary thermoelectric device. As a result, the thermoelectric conversion material can generate power continuously in inexpensive equipment, without exhaust gases resulting from combustion or the like, until the material deteriorates.
In light of these advantages, thermoelectric power generation holds promise as a technology that can contribute to tackling the grave problem of energy resource demand in the foreseeable future. In order to realize thermoelectric generation for universal situations, however, it will be necessary to supply large amounts of thermoelectric conversion materials having superior characteristics such as high thermoelectric conversion efficiency, heat resistance, chemical durability and the like.
Substances currently known to have high thermoelectric conversion efficiencies include intermetallic compounds. The thermoelectric conversion efficiency of intermetallic compounds, which can be used in the atmosphere only at temperatures not exceeding 300° C., is at most of about 10%. Also, thermoelectric conversion materials comprising intermetallic compounds contain often toxic elements and/or rare earth elements, among their constituent elements. This makes the use of such thermoelectric conversion materials in all-purpose applications difficult, in terms of safety and/or power generation costs.
For these reasons, thermoelectric conversion exploiting waste heat has not been realized thus far. It would thus be highly desirable to develop a material that should have high thermoelectric conversion efficiency, little toxicity, be composed of abundant elements and possess excellent heat-resistance, chemical durability and the like.
In recent years, Co-based complex oxides containing Ca, Bi and the like as “complex oxides having a high Seebeck coefficient and high electric conductivity” have been reported as materials having high thermoelectric conversion efficiency (Japanese Patent No. 3069701). In addition to exhibiting excellent thermoelectric characteristics, these complex oxides are composed of abundant elements, while exhibiting superior heat-resistance and chemical durability, and hence are expected to afford materials that are easy to realize.
Such Co-based complex oxides, however, have a p-type electric characteristic, and do not exhibit n-type characteristics. Both p-type and n-type thermoelectric materials are ordinarily required for building up a thermoelectric conversion element. That is because the efficiency of power generation obtained by combining a p-type material that generates negative thermoelectric power on a high-temperature side, and an n-type material that generates positive thermoelectric power on an opposite high-temperature side, more than doubles the generation efficiency achieved using exclusively a p-type or an n-type material. It is thus necessary to develop a material having high n-type thermoelectric conversion efficiency, as well as superior heat-resistance and chemical durability.
Ordinary examples of materials having excellent heat-resistance and chemical durability include oxides of transition metals and the like. Among such oxides, n-type thermoelectric conversion materials that have been studied include, for instance, TiOx “titanium oxide thermoelectric conversion materials” (wherein 1.89≦x<1.94, 1.94<x<2) and TixMyO2 “thermoelectric conversion material, thermoelectric conversion element and thermoelectric generator element using the same” (wherein M is at least one selected among Ta, Nb and V, and 0.78≦x≦1.0, 0.005≦y≦0.18, 0.01<y/x) (Japanese Laid-open Patent Application Nos. 2006-100683 and 2005-276959). However, a composition comprising N has not been studied yet.